a) Field of The Invention
The present invention relates to a method for quantitatively evaluating the tightness of the stator wedges of a large alternator. The invention also relates to an apparatus for carrying out this method.
In the following specification, the term "alternator" as it is used, designates any kind of hydroalternators and turbo-alternators, having either a vertical or a horizontal axis.
b) Brief Description of The Prior Art
As is commonly known, an alternator comprises a rotor and a stator defining an air-gap of a given width therebetween. In practice, the stator usually consists of superimposed metal plates in which are provided radial slots having outlets opening into tile air-gap. In each of these slots, at least one and preferably two stator coils are held by resilient holding means which may consist of a rubber bead or of a ripple spring which itself may consist of a long strip of composite material that is undulated either lengthwisely or transversally. The resilient holding means abuts, either directly or by the intermediary of a packing material and/or quoins, against a set of small stator wedges a few centimeters long, that are slidably inserted and retained in a set of symmetric grooves provided for such purpose in the metal plates of the stator, at the outlet of the corresponding slot. In large alternators like those powered by the turbines of a dam, there is a great number of these wedges, which are preferably made of composite material of the fiberglass type. Indeed, in large alternators, the stator can have as many as 500 slots along its periphery and each of them may be several meters high.
It is known that one of the main causes of forced interruptions of use of the alternators used in the hydroelectric power stations is due to defects in the stator coil windings, which are in turn due to vibrations of the stator coils. The vibrations may be caused by improper mounting of the wedges in their grooves or if the resilient holding means mounted between these wedges and the stator coils, lose some tightness in use. In such cases, there is indeed premature wear and a consequent major risk of short circuits. Therefore, it is a standard procedure to verify at regular intervals whether the stator coils are properly wedged.
The method currently employed for evaluating the stator wedge tightness condition of an alternator, i.e. detecting that the stator coils are properly wedged in place, consist of hitting with a hammer each of the stator wedges and detect by the sound produced whether the wedges are properly held in theft grooves by the pressure exerted by the resilient holding means located behind them. This method is simple and efficient but has two major drawbacks. Firstly, it calls for the rotor to be partially or completely dismantled so that an operator can effectuate the required hitting. Such can only be done on some important occasions. Secondly, this method is purely subjective, since the determination of the degree of "tightness" of the wedges according to the sound produced by the same is left entirely to the judgement of the operator who, well trained as he or she may be, may still make a mistake.
To tentatively obviate these two drawbacks, it has been proposed by ELIN-UNION A.G. or Austria, to hit the stator wedges without removing the rotor, by means of a micro-hammer that can be inserted into an air-gap wider than 10 mm. The sound produced by this hitting is recorded with a microphone and may thence be evaluated by an operator using headphones or by a computerized electronic sound analysis system. A similar method is also suggested in U.S. Pat. No. 4,970,890 issued on Nov. 20, 1990 to WESTINGHOUSE ELECTRIC CORPORATION.
In German laid-open patent application n.degree. DE-A-3,438,468published on Jun. 20, 1985, ELIN-UNION A.G. has also disclosed another method for evaluating the tightness of the stator wedges of an alternator without having to remove the rotor, which consists in verifying the stator wedge tightness of the resilient holding means holding the wedges in place, with a sensor having a thickness inferior to the width of the air-gap, the sensor including a mobile element actuable remotely by means of wedge-shaped members located at the extremities of a piston which is operated by a hydraulic fluid. In use, the sensor is inserted into the air-gap and placed in front of the wedge where an evaluation is to be performed, so that the mobile element of the sensor faces the wedge. The piston is then actuated and tile pressure "P" of the injected fluid is measured simultaneously with the longitudinal displacement of the piston which, thanks to the wedge-shaped members, causes a simultaneous transversal displacement of the sensor, whose value "L" of which is easily calculable. The curve giving the pressure "P" as a function of the displacement "L" that is thus obtained makes it possible to evaluate the stator wedge tightness of the resilient holding means.
Although this method is very efficient, it is, according to its manufacturer, usable only in air-gaps of a width greater than 20 mm. Moreover, such method is difficult to apply to large alternators, since it does not seem to provide means to properly position the sensor and its mobile element in front of each wedge to be verified.
A similar method has been proposed in U.S. Pat. No. 5,012,684 issued on May 7, 1991 to WESTINGHOUSE ELECTRIC CORPORATION. In this patent, there is disclosed a method for evaluating the stator wedge tightness of an alternator using a sensor especially designed to measure the elasticity of the rubber bead or ripple spring holding the wedges, by measuring the displacement of this rubber bead or ripple spring as a function of the exerted pressure. In this particular case, the radial slots of the stator in which are located the stator coils must be provided with two sets of grooves: one for the stator wedges and another one in which the sensor can be inserted. Such construction renders the sensor useless unless the alternator is of the above type.
WESTINGHOUSE ELECTRIC CORPORATION has also proposed another kind of sensor insertable in the air-gap of an alternator to measure the deflection of the ripple springs holding the stator wedges. The sensor which forms the subject matter of U.S. Pat. No. 5,020,234 issued on Jun. 4, 1991 is very complicated from a structural standpoint. Moreover, even if an extra set of grooves is not required, the wedges must be provided with perforations, since the sensor is devised to measure the deflection of the ripple springs by contacting the same with a rod which must be aligned with the selected perforation using a miniature television camera to do so.